Bacterial growth in vivo is the primary requirement for pathogenicity, and growth is limited by nutrient availability, which can change during the course of an infection. Iron that is essential for virtually all bacterial growth is actively withheld by mammalian defense mechanisms. Bordetella pertussis and Bordetella bronchiseptica are highly genetically related bacterial species that cause respiratory infections in mammals. B. pertussis is the agent that colonizes the human respiratory epithelium to cause whooping cough, or pertussis, and B. bronchiseptica infects a wide range of nonhuman mammals. For iron retrieval, B. pertussis and B. bronchiseptica produce and utilize the siderophore alcaligin, and can also use the enterobactin siderophore and host heme compounds. Under iron starvation conditions in vitro and in vivo, each system is positively regulated by the action of a distinct transcriptional regulator. These regulators are activated by the cognate iron source acting as the inducer, allowing maximal expression of the genes required for its utilization. Previous studies showed that these iron acquisition systems are important for B. pertussis infection. They are differentially expressed in a host, and their expression patterns reflect changes in iron source availability during infection. The alcaligin system is important for B. pertussis growth during the entire infection, enterobactin utilization is most important to successful early infection and the heme system is required only at later stages of infection. Our long-term interests are to further our understanding of nutrient acquisition as a key virulence determinant in Bordetella species. The proposed research is focused on extending our knowledge of Bordetella iron assimilation to characterize newly identified iron transport systems and determine their importance to growth in the host. In this research, we will define the role of key Fbp system components in Bordetella utilization of ferric siderophores and characterize an apparently novel mechanism for utilization of ferrous iron. Lastly, we will investigate the means by which Bordetella cells use host neuroendocrine stress hormones to obtain the iron from transferrin and lactoferrin.